Ozone above Indian Ocean Linked to African Lightning

BOULDER --
In one of the first studies to trace lightning's chemical
impact across thousands of miles, a team of atmospheric chemists has
connected a region of elevated ozone levels in the eastern Indian Ocean
with lightning produced in Africa. The results will be presented
December 6 at the American Geophysical Union conference in San Francisco
by Louisa Emmons, a visiting scientist at the National Center for
Atmospheric Research (NCAR). NCAR's primary sponsor is the National
Science Foundation.

Emmons and colleagues examined a set of ozone data collected over four
years between Japan and Antarctica for their paper, "Evidence of
Transport Across the Indian Ocean of Ozone Produced from Biomass Burning
and Lightning" (AGU paper A12D-11). Her coauthors are Didier
Hauglustaine (France's Centre National de la Recherche Scientifique),
Michael Newchurch (University of Alabama at Huntsville), Toshi Takao and
Kouji Matsubara (Japan Meteorological Agency), and Guy Brasseur (NCAR).
The research was funded by the National Aeronutics and Space
Administration.

Lightning is known to produce nitrogen oxides (NOx) within
thunderstorms. These chemicals may react with others in the presence of
sunlight to produce ozone. Until now, most related studies have focused
on measuring the production of NOx in the immediate vicinity of storms.
However, the ozone produced has a long lifetime in the upper troposphere
and thus could be carried over long distances. According to Emmons and
colleagues, ozone from storms across southern Africa is being
transported by the subtropical jet stream to Australia.

Ozone measurements between 2 and 6 miles in altitude (3-10 kilometers)
over a large part of the eastern Indian Ocean were as high as 80 parts
per billion, similar to a polluted day in a U.S. city and several times
more than normal levels, says Emmons. To analyze the source of this
ozone, she and colleagues used a new computer model of atmospheric
chemistry called MOZART, developed at NCAR by Brasseur and Hauglustaine.

Results from MOZART indicate that the ozone did not descend from the
stratosphere, the most obvious source. Another possible source was the
burning of forests and grasses upwind in Africa. When biomass burning
was removed from the model calculations, ozone levels remained high, but
when African lightning was removed, the ozone levels dropped
significantly. The MOZART results are consistent with the observations
above.

"Although there are uncertanties in the model results," says Emmons,
"they indicate that lightning has a far-reaching and significant impact
on tropospheric chemistry."

The University Corporation for Atmospheric Research, a consortium of
more than 60 universities offering Ph.D.s in atmospheric and related
sciences, manages NCAR.

filename: mozart1.tif

Ozone levels from a standard run of MOZART, including biomass burning
and lightning emissions, in parts per billion (ppb) on a single day at
the altitude of 300 millibars (about 6 miles). Highest ozone levels
(purple) are over Africa, where the ozone is being produced, but high
values extend across the Indian Ocean to Australia.

filename: mozart2.tif

Ozone levels from a standard run of MOZART, including biomass burning
and lightning emissions, in parts per billion (ppb) on a single day
along a vertical cross section (N-S) at longitude 100 degrees E. The
area of enhanced ozone between 20 and 30 degrees S latitude and 3 to 10
kilometers altitude is believed to be related to African lightning. Fi

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Programs are operated by UCAR under the sponsorship of the National
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Prepared for the web by Jacque Marshall